In treating cardiac arrhythmias, it is well known that the ability to sense electrical activity in both chambers of the heart can greatly improve both the range of therapies available, as well as the effectiveness of those therapies. In a typical endocardial pacemaker, a first catheter lead having an electrode at the distal end is positioned to pace and sense the ventricle, and a second catheter lead having an electrode at the distal end is positioned to pace and sense the atrium. In a defibrillator system, a defibrillation electrode is added to one, or both, of the catheter leads to deliver high voltage defibrillation countershocks to the ventricle and/or atrium. While the use of two separate endocardial catheter leads for the ventricle and atrium has certain advantages, primarily in terms of independently optimizing electrode location within each chamber of the heart, the obvious disadvantage of this approach is that it requires two separate catheter leads to be inserted into the heart.
Although the idea of providing a single endocardial catheter lead for treating both the atrium and ventricle has long been known, to date, there has been little success in achieving this objective. Prior attempts to implement a single-pass, endocardial catheter lead can be classified into two categories: (1) distal end electrode configurations, and (2) mid-lead electrode configurations. In the first category, the electrodes for pacing and sensing each chamber of the heart are located on two separate distal end portions of the same catheter lead. While this approach allows for optimum location and attachment of each distal end electrode, it has been found that the separation of the catheter lead into two branches or legs within the heart increases the opportunity for thrombus to develop resulting in the possible sloughing off of emboli, particularly at the point at which the two branches of the catheter lead join together. In the second category, the electrodes for pacing and sensing the ventricle are located on the distal end portion of the catheter lead and the electrodes for pacing and sensing the atrium are located mid-lead along the body of the catheter lead within the atrium. While this approach avoids the thrombus problem of two branch catheter leads, it has been difficult to develop long-term effective pacing and sensing using mid-lead electrodes in the atrium.
Presently, most of the mid-lead electrode endocardial catheter leads which have been commercially released utilize a free-floating approach for positioning the electrodes within the atrium. Examples of this approach are shown in U.S. Pat. Nos. 4,585,004, 4,892,102, 4,917,115, 4,962,767, 5,127,403 and 5,172,694, all of which describe different ways to optimize the positioning of the free-floating electrodes in order to sense the P-wave electrical signals associated with the atrium. While free-floating electrodes can provide good sensing, the lack of constant chronic contact with the wall of the atrium decreases the long-term effectiveness of these electrodes.
Examples of other mid-lead electrode endocardial catheter leads which utilize some form of biasing to press the electrode against the atrial wall are shown in U.S. Pat. Nos. 4,154,247, 4,401,126 and 4,627,439. There are significant challenges, however, in designing a mechanical biasing arrangement that will prove effective over the long-term as the atrial electrodes are subject to movement due to cardiac activity, respiration and patient movement or orientation.
A endocardial catheter lead with mid-lead electrode which utilizes a barbed attachment mechanism and porous electrodes is shown in U.S. Pat. No. 4,497,326. This approach is quite similar to the various techniques known for securing a distal end electrode, such as are shown, for example, in U.S. Pat. Nos. 3,737,579, 4,444,206, 4,620,550, 4,972,849, 5,074,313, 5,330,520 and 5,423,884. All of these patents show some form of active fixation mechanism (e.g., a hook, barb, tine, corkscrew or fin designed to penetrate the heart wall) for the distal end electrode. While this approach would seem to solve the problems of the free-floating approach and the mechanical biasing approach, the tension and stress induced on active fixation mechanisms at the mid-lead electrode by the continual movement of the more securely anchored distal end electrode can potentially cause abrasion or scarring of the atrial wall tissue.
The use of a passive fixation mechanism, instead of an active fixation mechanism, for securing distal end electrodes, is shown in U.S. Pat. Nos. 4,374,527, 4,407,303, 4,573,480 and 4,465,079. In these patents, insulating material which extends from the orientation of the catheter lead proximate the distal end of the catheter lead is utilized to encourage fibrosis so as to anchor the distal end electrode in a manner similar to a distal end active fixation mechanism. All of these patents, however, are limited to single chamber catheter leads having distal end electrodes.